1. Field of the Invention
This invention relates generally to color presentation by different devices--for example different printers, cathode-ray tubes (CRTs), and liquid-crystal displays (LCDs)--and more particularly to coping with divergent color gamuts of such color-presenting devices, when color specifications developed with one device are used to control color presentation on another device.
An exemplary application is in controlling color presentation by a printer, based on color specifications developed using a CRT computer monitor.
2. Related Art
A. Coexistent machine color spaces--The use of color monitors for computers has accelerated the need for color printers which can produce a satisfactory printout of what is displayed on the screen. As suggested above, this is one important instance of coordinating color appearances produced with two different physical systems.
With all such equipment, determining what constitutes a "satisfactory" color presentation--for instance, color printout on a printing medium such as paper--is often quite problematic.
Part of this problem arises from the subjective nature of color. Color is a sensation produced by combined effects of light, objects and human vision. A particular color or combination of colors may be appealing to one person while at the same time being offensive to another.
Another part of the "satisfactory"-color definitional problem arises from different color technologies used in computer monitors, color printers, and other color-presenting devices--such as broadcast or tape-recorded video. In general these technologies diverge dramatically.
For example, color presentation by CRT computer monitors and television sets is based on a color gamut defined by red, green and blue (RGB) CRT intensities. Color presentation by printers such as inkjet printers is instead typically based on a color gamut defined by cyan, magenta, yellow and black (CMYK) printed-page colorants.
The RGB color intensities of CRT screens are combined together in an additive way by mixing red, green and blue light rays from a first class of physical substances--namely phosphors--to form a first variety of different colors. The CMYK components of color inks, a second and entirely different class of physical substances, are applied to media in different combinations in a subtractive way to form a second variety of different colors; and the three chromatic elements CMY are only nominally the complements of the RGB intensities.
Various different color-management techniques have been used to provide some form of coordination between, for instance, the colors viewed on a computer monitor and the colors printed by a specific printer using a given ink formula on a particular type of printing medium. Modernly, most such control regimes are implemented in computer software or device firmware, or both.
Some color-matching technology has been incorporated into printer drivers, and in some such cases partially into accompanying lookup tables. A driver provides a translation interface between--on one hand--a particular computer operating system, and/or application software running in the computer, and on the other hand a color printer which acts as a hardcopy output device.
Portions of this document refer to an "input device" or "source device". By this is meant a device in reference to which a set of color-image specifications has been developed--not necessarily a device within which specifications literally originate.
Quite commonly, for example, a person watching a CRT screen (here serving as the input device) creates or modifies specifications for the colors, as seen on the screen, of various image portions, respectively. A printer or other device may be used instead of a CRT as the input device, even though naturally CRT screens are in general much faster and so in most cases preferred for this purpose--and conversely a CRT may be an output device.
It will be clear to readers who are skilled in this field that the CRT screen or other input device is not literally the source of the specifications. Rather, the source of the specifications is usually the person, typically in interaction with some apparatus or computer program--often a so-called "applications" program.
Such apparatus or program receives the person's commands, translates them into display-device signals and causes them to be displayed on the CRT screen. The physical properties (or so-called "characterization") of the CRT enter into the determination of how the specified colors will appear.
Once created or modified, or both, specifications are commonly directed into control systems for operation of another device--the "output device" or "destination device"--often a printer. Thus in many cases both the "input device" and "output device" in common are recipients of the specifications, in the form of data, electrical signals etc.--and the differences between physical properties or "characterizations" of the two devices in general give rise to differences between the resulting color appearances.
Terms such as "source device" or "input device" in this document thus encompass color-presenting devices which may be used by a person in conjunction with developing color-image specifications. These terms also, however, encompass devices that actually do serve as the source of the specifications.
An instance of this which is of particular interest is an applications program or system which leaves--as some do--the CRT signals or data essentially unaltered. In such a case the information in substantially the same form as used to control the CRT is treated as, or may be regarded as, the color specifications.
B. Pipe-through systems--In designing control systems for early apparatus, a classic assumption was sometimes used that the red, green and blue (for example) of one system were compatible, or consistent, with the red, green and blue complements of the cyan, magenta and yellow (for example) of another system. These assumptions are grossly incorrect, but had several interesting and useful properties.
First, the assumptions and resulting conversions were simple and required little understanding of relationships between the different color-production mechanisms used in the different systems. In particular these assumptions sidestepped all inquiry into the color gamuts respectively available with the two systems, and so into the relationships between those gamuts.
Therefore these assumptions were very easily implemented. In essence the RGB data were simply piped through to form CMYK data--but with elementary conversions based on the previously well-taught oversimplifications of so-called "complementary" colors; and also with some side calculations to derive black-ink (K) quantities.
Second, seen from the more-modern perspective of gamut relationships, these assumptions had the technical or theoretical benefit of in essence forcing both gamuts into concurrence, thus completely using both color spaces. They perfectly superposed the source- and output-device gamuts, thus producing colors with--potentially--a full range of vividnesses and lightnesses.
Third, viewed from a simple perceptual standpoint, these assumptions did produce color presentations for output devices (e. g., usually printers) that were usually distorted in relation to the colors presented by the input devices (e. g., usually CRTs). The relationship was generally quite poor--suffering from severe errors in hue and chroma, usually obscured by even more severe errors in lightness.
Because apparatus users found these lightness errors unacceptable, system designers next proceeded to incorporate lightness/darkness adjustments into the otherwise piped-through signals. These adjustments were sometimes labeled with such words as "lightness" and "darkness".
Sometimes they were instead called adjustments in "contrast" and designated by the corresponding parameter ".gamma." (a lower-case Greek letter "gamma")--a classical variable in photography, photolithography and photometrics. In either case these adjustments were often actually implemented in the classical manner as .gamma. slopes in a logarithmic response space.
In any event, lightness/darkness corrections generally took the form of separate adjustments--generated painstakingly through trial-and-error--to red, green and blue input signals. Through these efforts, lightness as presented by an output device could be brought into an acceptable range of relationships with that presented by an input device.
Another result, however, as the residual of this process was to reveal the theretofore-unaddressed deviations in hue and chroma. With lightness errors under relatively better control, these chromatic errors became far more conspicuous--and they are much more difficult to master.
C. Overlapping gamuts--In later apparatus some effort was addressed to controlling the relationship between these perceptual chromatic elements (hue and chroma) of input- and output-system colors. This effort generally took the form of attempts to match the appearances of the two.
This step entailed analytically characterizing the two devices--e. g., CRT and printer--in some common perceptual space. At this point the physical reality of the differences between the two sets of colors and the two device gamuts came out, disclosed by the perceptual frame of reference.
Now for the first time it became meaningful to consider the matter of dealing with gamut divergences. In general the gamuts of any two different devices fail to be congruent, and in particular often are overlapping--in all three dimensions of color space.
Thus typically each of the two gamuts extends to more-vivid colors, in some parts of the lightness ranges and some parts of the hue spectrum, than does the other gamut. Moreover, typically one gamut extends to lighter or to darker colors, or to both lighter and darker colors, than does the other gamut.
This overlap phenomenon implies, for example, that some colors which can be displayed on a CRT simply cannot be printed on a printer. It also has a converse implication (which will be introduced shortly)--but the unavailability of printer representations for some CRT-representable points is a particularly demanding matter.
It is demanding because color specifications developed by a person operating a CRT will in general include data points that are outside the printer gamut--but yet must be processed in some way. Thus system programmers, whether they wish to make color-relationship decisions or not, are forced to instruct systems what to do with those out-of-gamut points.
Faced with this necessity, system designers devised one or another scheme for translating the furthest-out-of-gamut CRT color into the interior of the printer gamut. A natural concern in specifying such a scheme is how to retain some degree of consistency in dealing with all colors--in other words CRT color-data points that are within the printer gamut, as well as different points that are outside it.
This idea of consistency is very important, particularly in presenting color images that include pictures, intended to appear realistic, of physical objects. Pictures of objects often derive a degree of realism through preservation of color relationships between colors of physically nearby image elements, such as for instance adjacent portions of subtlely-shaded clouds, or of the surfaces of curved objects. Another way to refer to preserving such "relationships" is to speak of retaining "information content".
A classical example of the relationship-preserving or information-content-retaining challenge has been denominated the "gum-ball problem". This phrase conjures the difficulty of realistically presenting a color picture of an object whose portrayed curvatures extend through a full range of values from flat-on to tangential.
As is well known, realistic portrayal of curvatures generally requires reproduction of a continuum of surface shadings--without which surfaces tend to appear, literally, flat. Attaining such realistic presentation is particularly difficult for a vividly colored object such as a gum ball.
The necessary shadings needed to realistically portray curvatures are especially demanding for vividly colored objects because these colors are near the peripheries of overlapping-gamut systems. In particular, often, these colors are within the source-device gamut but outside the destination-device gamut.
The difficulty arises from the need to somehow preserve as much as possible of the relationships between these many different shadings found in the source-device gamut--even when they are all outside the output-device gamut. The question then is how to represent, within the capabilities of an output device, relationships between many different vivid-color appearances that are all, by definition, outside those capabilities.
D. Gamut compression--One standard solution to these concerns heretofore has been to make a proportional adjustment to the chroma, or distance from the central lightness/darkness axis of the three-dimensional color-solid gamut, for all CRT-defined points. A common variant of this technique is proportional adjustment to both chroma and lightness--the latter being represented as position projected along that central axis.
The chroma-adjusting proportionality factor in such systems is less than unity, and is generally constant. Usually the proportionality factor is constant throughout the entire three-dimensional gamut; possibly in some systems, however, that factor is at least constant throughout each so-called "hue page", or defined grouping of hue pages.
(A hue page, as considered within any single specified color space, is a vertical plane extending radially outward from the central color-space axis of lightness/darkness, and representing the available variations of chroma and lightness that make up all the available colors of a selected single hue. As between different color spaces, the hue-page concept has important limitations which are discussed in section [d] below.)
The selection of a constant proportion throughout the color space may be an intuitively satisfying choice, since its physical implication is in theory a mapping of all colors with no change in lightness or hue, only a change in vividness--and since, furthermore, at least in an analytical sense this vividness change is applied consistently to all the colors in the CRT gamut.
Thus the various colors retain nearly their original lightness and hue, and also retain their mutual relationships in a comparative way. For example, if a first color on the CRT screen is originally darker, bluer and more saturated than a second on the CRT screen, then in principle the resulting, first color printed on paper should likewise be darker, bluer and more saturated than the resulting second color printed on paper--and a like relationship should be exhibited for any selected pair of colors.
One variant of this proportional-adjustment technique is to incorporate accompanying lightness adjustments, or often lightness/chroma tradeoffs (which themselves may or may not operate on the same principle of proportionality). These adjustments are provided to avoid some intuitively unsatisfying results of proportionally adjusting chroma exclusively.
All such proportional-adjustment techniques, sometimes known as "gamut compression", nevertheless have drawbacks. One relatively minor problem is that the theoretically consistent character of the transformation is not in general observed for real systems; in other words, the vividness compression is sometimes perceptually nonuniform.
More serious and fundamental is the fact that gamut compression artificially restrains the vividness of almost all the colors printed. The only colors in the CRT gamut which are printed near the full vividness of which the printer is capable are (1) the CRT color which is furthest out of the printer gamut and (2) the near-neighbors of that CRT color.
Even that furthest-out-of-gamut color is, by definition, very significantly reduced in vividness from what is seen on the CRT screen--but this cannot be helped because the printer gamut at this point does in fact limit the available vividness. Everywhere else in the printer gamut, however, the capability of the printer to present saturated, vivid colors is essentially discarded.
Thus in essence all the colors in the CRT screen gamut are synthetically and systematically reduced to the lowest common denominator, so to speak, of the printer gamut. One result has been to create among many users a misapprehension that CRT screens are capable of displaying colors vividly whereas CMYK printers are not.
As to printers the contrary is true--but the colors which printers can print vividly are different colors from those which CRT screens can display vividly. This is the "converse implication" mentioned parenthetically above.
To state that converse implication directly: some colors which can be printed on a printer simply cannot be displayed on a CRT. Evidence of this fact heretofore was, in effect, inadvertently concealed from many users by the popular technology of color-matching and gamut compression.
Thus in inventing gamut compression to solve the earlier problems of lightness and hue shifts in printed colors, relative to CRT-displayed colors, system designers created a new problem: printed colors which failed to make use of the printer gamut and so appeared--in comparison with CRT displays--drab, lifeless, boring, and generally disappointing to users of color-presenting apparatus.
Gamut compression, while serviceable in many contexts, thus failed to provide a technique for presenting such colors in a manner that is both fully useful (retaining information content, to a reasonable extent) and perceptually vibrant.
An earlier passage in this discussion posed the problem of "how to represent, within the capabilities of an output device, relationships between many different vivid-color appearances that are all, by definition, outside those capabilities." To that expression of the problem may now be added the additional constraint that the desired representation should at the same time preserve, to the extent possible, the vividness of the original colors.
The gum-ball problem, after all, is easily solved in the limiting case of black-and-white pictures--or whenever vibrant color is of little consequence. The challenge resides in concurrently retaining the vividness of the bright colors used in making the balls.
E. Surface scaling--Another line of effort has focused more emphatically on this latter constraint. Unfortunately, as will now be seen, the result has been to sacrifice a large part of the information-preserving or relationship-preserving concern.
Here the technique entails generally maintaining essentially unchanged the colors within the common portions of the input and output gamuts. Adjustments are applied exclusively--or almost exclusively--to those specified colors which are within the source-device gamut but found to be outside the destination-device gamut.
These adjustments consist of mapping, or moving, all those specified colors to some points along the destination-device gamut surface. In this line of development, known as "surface scaling", various rules have been adopted by various workers for selecting the new point along the output-gamut surface to assign to each given out-of-gamut color.
At the outset it will be understood that considerable loss of information is necessarily inherent in any such exercise, for at heart it consists of mapping areas into lines. Nonetheless the effort is entirely meritorious because retention of vividness is very important.
In fact, for many users vividness of the final presented color is at least as important as, and sometimes more important than, the retention of intercolor relationships and thus information conveying, e. g., spatial curvatures of objects portrayed. A major issue, then, is the extent to which the appearance, or even illusion, of seeming to retain such relationships can be sustained.
One straightforward kind of surface scaling is mapping colors to the output-gamut surface at constant lightness--in other words, displacing each out-of-destination-device-gamut point along a horizontal (constant-lightness) line to the intersection of that line with the gamut boundary. Intellectually this may seem the optimum arrangement, since it preserves lightness, and operationally may be appealing in that it is very simple to implement.
Unfortunately, however, because of the relative geometries of gamut boundaries, this approach tends to preserve relatively very small fractions of the relationships between out-of-destination-gamut colors. In other words this approach typically retains very small fractions of the information needed to distinguish different parts of objects.
Consideration of typical device-gamut shapes, particularly within hue pages, reveals why this is so. Device gamut boundaries as represented in perceptual spaces, in the usual lightness-vs.-chroma hue-page geometry, typically lie neither near-horizontal nor near-vertical but rather middlingly in between.
Hence a horizontal (along-constant-chroma) displacement of out-of-destination-gamut points approaches the destination-gamut boundary along an acute angle. The result of using this particular form of area-to-line mapping is to map a very long, horizontal line (within the out-of-gamut areas) into each point on the destination-gamut surface.
Similar objections may be lodged against a competing candidate mapping--namely, along vertical (constant chroma) lines. While perhaps not as often sacrificing as much information content in general, such a mapping tends to collapse multiple values of lightness onto each gamut-boundary point.
This particular sort of information deformation is particularly troublesome: first, it flattens apparent shapes of curved surfaces. Second, it also draws attention to itself by introducing relatively severe lightness shifts per se--and, as previously suggested, shifts in this color dimension are particularly noticeable and objectionable.
Mindful of these relationships, some workers choose instead to map each out-of-gamut color point to the nearest point on the gamut boundary. For much of the out-of-gamut area this amounts to displacing the color points along a normal to the boundary, which--considering typical overlapping-gamut geometries--can be seen to minimize the length of displacement.
In this way, still for much but not all of the out-of-destination-gamut area, this mapping geometry tends to maximize the extent of relationship/information retention. In certain areas of the hue page, however, the mechanism leading to this benefit tends to break down.
Those include, first, the areas in which there is no normal to the gamut boundary, and second the areas which are adjacent to those areas. Study of hue-page graphs for typical overlapping-gamut situations will reveal that these conditions can usually obtain only in relatively small hue-page regions.
These are the regions near the maximum-saturation point and the lightness/darkness-extremum points for the destination gamut. From the localization and smallness of these effects it might be supposed that they are correspondingly of little consequence.
To the contrary, as a practical matter it is found that the quality of mapping in these areas is particularly important to the user. Areas near the maximum-saturation point are of particular concern to users generally, as the earlier presentation of the "gum-ball problem" makes evident.
Mapping performance in areas near the extrema of darkness and lightness implicates the capability of an image-presentation system to preserve image information or detail only in very dark and very light parts of the image. As such, image appearance in these selected areas may seem an obscure criterion.
Such a criterion may perhaps be of greatest concern only to relatively more-discerning users, but of these there are plenty. Furthermore, classical evaluations of image-presenting technologies from oil painting through photography and photolithography have concentrated attention upon these criteria.
With this in mind, one may go on to notice an important phenomenon of surface scaling by application of any algorithm that maps to the nearest surface point. In the out-of-gamut regions near the three extremum points, those extremum points tend to act as lightning rods, collecting all the colors from diverging, sector-shaped regions outside the destination gamut.
It is fair to conclude that surface-scaling techniques of the to-the-nearest-boundary-point type leave considerable room for improvement with respect to image-presenting quality in these critical regions of the color space.
F. Data storage and processing requirements--Here-to-fore the amount of data needed for useful characterization of hue pages in machine or perceptual color spaces has bordered on the massive.
In some cases characterizations have taken such forms as, for instance, point-by-point perceptual chroma and lightness values corresponding to RGB data triplets or other machine-space data. Typically these values are tabulated for each point in a grid covering the entire interior of a hue page (or other color-space element) of interest.
In other cases characterizations have taken the form of point-by-point descriptions of upper and lower gamut boundaries for each hue page of interest. Here the amount of data required is less than in the full-interior characterization just mentioned, but still unwieldy.
Even with such multipoint characterizations, considerable operating time typically must be devoted to interpolation, in three color-space dimensions, between the table entries. Thus operating-time cost is added to the cost of data storage.
In still other cases perhaps there has been put into use, for characterizing chroma and lightness within a hue page, some multiterm polynomial expansion that satisfies boundary values for each hue page of interest. This may be an improvement, but has some tendency merely to replace near-massive data storage with near-massive data processing--a substitution that saves disc space only at the cost of time and throughput penalties.
For some purposes, detailed and accurate lookup tables, or calculations good to three or four places, seem serviceable and in some cases perhaps even unavoidable. A methodology, however, heretofore has been severely needed for at least mitigating this brute-force approach to machine- and perceptual-space characterizations, wherever extremely high precision or accuracy is not fundamentally necessary.
G. Summary--Thus there has been heretofore a need for a refined color-management technology which somehow permits use of the full color-saturating capability of printers. It is desirable that such a technology at the same time preserve at least some of the acknowledged benefits of color-matching, and of its perhaps most-popular implementation, gamut compression.
For example, again with respect to CRT/printer systems, the related art provides no system for printing well-saturated color in conjunction with:
at least approximate preservation of the lightness relationships in a CRT-displayed image; or PA1 preservation of what may be called "information"--meaning the discrimination between initially adjacent colors, with at least some preservation of the hue and of the relative, if not absolute, lightness and vividness magnitudes--in a CRT-displayed image. PA1 at least approximate preservation of the lightness relationships in a corresponding image produced by a source color-presenting device; or PA1 preservation of information in the source color-presenting device. PA1 This method includes the step of interpreting relative-position values of color parameters, within a first color element of at least two dimensions, as also being relative-position values of corresponding color parameters within a second color element of the same number of dimensions. By "color element" is meant a three-dimensional relationship in color space, or a two-dimensional relationship ("surface") in that space. PA1 As will be seen, this step entails establishing certain color-parameter values by manipulations based upon the physical significance of the first color element--and then treating or using those parameters as if their physical significance were related to the second color element. It may thus be regarded as a renaming or assigning step, and as will be seen has application in a number of different situations and for a number of different purposes. PA1 The method also includes the steps of using the relative-position values within the second color element to derive signals for controlling the color-image presentation device; and then applying the signals to operate the color-image presentation device for presentation of the color image in viewable form. PA1 With respect to this main aspect of the invention, some particularly preferable embodiments may be mentioned here. In one group of these embodiments, the first color element is expressed in terms of a perceptual color space, and the second color element is expressed in terms of a device color space. PA1 In this case the interpreting step can be used to produce the physical effect of translating perceptual-space values within the first color element into device-space values within the second color element. Very large savings in data storage or processing time, or both, can be obtained in this way. PA1 Still with respect to the same main aspect of the invention but in another group of embodiments, the first color element is expressed in terms of a first perceptual color space--and the second color element is expressed in terms of a second, different perceptual color space. In distinction to the first group of embodiments introduced immediately above, here both elements are in perceptual spaces. PA1 In this case the interpreting step can be used to produce an entirely different physical effect, namely matching of the first color element to the second color element to provide substantially full use of the second color element. For example the first color element may be at least part of a color gamut of a first presentation device, and the second color element at least part of a color gamut of a second, different presentation device; this use of the interpreting step matches the gamuts to provide substantially full use of the color gamut of the second device. PA1 This method includes the step of receiving or developing color-image specifications defining a desired image. At least some of the specifications are expressed in terms of a hue page. PA1 Another step is finding vertex coordinates for a straight-line-bounded figure representing a simplified shape of the hue page of interest. Yet another is characterizing the hue page in terms of the vertex coordinates. PA1 Further steps are expressing the specifications with reference to the vertex coordinates, and then manipulating the specifications as thus expressed with reference to the vertex coordinates. The method also includes the step of then applying the manipulated specifications to operate the presentation device for presentation of the image in a form viewable as a color image--including control of colors as seen in the image. PA1 Further particularly preferable embodiments of this main aspect of the invention include employing extremum (minimum and maximum) lightness points common to all hue pages. When combined, for example, with use of the triangular simplification mentioned above, this particular simplification leads to single-point characterization (lightness and chroma for maximum chroma, or normalized lightness for normalized saturated chroma) for each hue, plus a two-point lightness characterization for the three-dimensional gamut. PA1 This method includes performance of steps that have the effect of defining a point within the device gamut, and then constructing a line that joins an out-of-gamut color point with that defined point. It also includes the step of then determining the intersection of the constructed line with the gamut boundary. PA1 In addition the method includes the step of scaling the out-of-gamut point to that intersection to determine a surface-scaled color specification corresponding to the out-of-gamut point. Thus as can be understood this invention provides a new kind of surface scaling by radial movement toward (but not to) a point within the gamut; in view of this radial pattern of data displacement the internal defined point may be conveniently denominated a "pivot point". PA1 The method is based upon initial color specifications developed for use in an input color-presentation device that has a different at least partly known color gamut. These initial specifications are expressed in terms related to control of the input device. PA1 This method includes the step of establishing a first relationship between input color specifications, expressed in terms related to control of the input device and corresponding colors, expressed in perceptual terms, presented by the input device. It also includes the step of establishing a second relationship between colors, expressed in perceptual terms, and output color specifications expressed in terms related to control of the output device. PA1 Further the method under discussion includes the step of combining the effects of the first and second relationship to determine a third relationship between input and output color specifications, respectively in terms related to control of these two devices. In this method, the third relationship maps an input color specification for control of the input device into an output color specification for control of the output device, such as to present a perceptually corresponding color on both devices. PA1 In addition the method includes the step of applying the initial specifications, altered by use of the third relationship, to operate the output device. The output device is thus used for presentation of a color image in a form viewable as a color image. PA1 The method includes the step of receiving the initial specifications, expressed in terms related to control of the input device, for a particular desired image. It also includes the step of referring to a tabulation which maps input color specifications for control of the input device into output color specifications for control of the output device--such as to present perceptually corresponding colors on both devices. PA1 Further this method includes the step of finding, from the tabulation, new color specifications for presentation on the output device--of colors perceptually corresponding to the initial specifications, for the particular desired image. An additional step of this method is applying the new color specifications to operate the output device for presentation of the image in a form viewable as a color image. PA1 Here the method includes the step receiving or developing a color specification for a portion of the image expressed in terms for control of the source device. The method also includes the step of performing on the color specification a transformation that has the effect of performing these substeps: PA1 The method also includes the step of then applying the new specification to control printing of the particular image portion by the printer. PA1 Now in particularly preferable embodiments of this main aspect of the invention, this method is used for scaling from one hue page in perceptual space to another--not necessarily in the same perceptual space. In some such embodiments hue preferably is used as a correlating factor. PA1 In other particularly preferable embodiments, the described method is used for scaling between hue pages in two machine spaces (e. g., monitor and printer)--in either direction--or between a perceptual and a machine space, still in either direction. In yet other preferable embodiments, perceptual proportionality is used as a means of scaling a data point from one hue page to another. PA1 For purposes of defining this method, it does not matter whether the image portion is an individual pixel or group of pixels, or an area printed as a solid--or even composites of these possibilities. In other words the image portion may take any of these forms, or still others. PA1 The printing of the image portion is based on initial color specifications for use in a display device that has color gamut likewise at least partly known. The method includes the step of performing on these initial specifications a transformation that includes the effect of performing these substeps: PA1 Here the "interpreting" substep may take the form of implicitly renaming variables. In other words this substep simply starts with values derived as having a first physical significance, and then uses those values on the understanding that they also have a second, different physical significance. PA1 This central function will be demonstrated with full specificity in the detailed-description section of this document. The method also includes the step of then applying the printer signals, to control printing of the particular image portion by the printer. PA1 In this description or definition, as elsewhere in this document, reference is made to a transformation that "has the effect of" performing the stated substeps. This language is used so as to encompass practice of the invention by either of two equivalent procedures. PA1 One of these procedures is actual performance of all the stated substeps, concurrently with preparing to print the image portions or the entire image. The other equivalent procedure is a well-known software and firmware technique of instead referring to lookup tables. PA1 This technique substitutes--for actual-performance of all or some of the stated substeps--reference to one or more lookup tables. The entries in the lookup tables are preassembled by performing functions equivalent to the stated functions, or to subgroupings of those stated functions. PA1 The printing is performed with a color printer that has a known color gamut, and is based on initial color specifications for a display device that has a known color gamut. This method includes the step of receiving or developing, for a particular portion of the image, the initial color specifications expressed in terms of the display-device gamut in a color space related to operation of the display device. PA1 This method also includes the step of performing on these specifications for the particular image portion a transformation that has the effect of performing these substeps: PA1 In addition the method of this aspect of the invention includes the further step of then applying the new color specifications, expressed in terms of the printer gamut and the printer space, to control printing of the particular image portion by the printer. PA1 Merely as a shorthand summary or tutorial aid and not as a definition of the invention, the sub-steps and steps described above progress successively thus: PA1 As can be seen, the "employing" substep mentioned above includes an "interpreting" function which is related to that in the main aspect of the invention discussed just before this one. The present method thus has the effect of a sequence of functions that includes--in the "using" substep--finding "relative position" perceptual values of hue and chroma. These are the "first intermediate" values. PA1 Most typically, this function includes shifting and warping the "absolute position" perceptual values of lightness and chroma obtained in the earlier "determining" substep. Other ways of finding the relative-position values, however, may be substituted equivalently. (Commonly as an approximation, where the display-device-gamut intercept on the lightness axis is relatively small, shifting may be omitted.) PA1 Then in the "employing" substep these first-intermediate values are subjected to an "interpreting" function that may be substantially a renaming function as explained above in connection with the just-previously-discussed aspect of the invention. PA1 More specifically, the tables are for use in printing such an image with a color printer that has a color gamut, likewise at least partly known. The lookup table is prepared so that the tabulated values correspond to performing steps including: PA1 The interpreting step is substantially as set forth above in regard to an earlier-discussed major facet of the invention. PA1 The printing is performed with a color printer that has a known color gamut, and is based on color specifications for a display device that has a known color gamut. This method includes the steps of: PA1 The comments provided earlier to aid in understanding the previously discussed aspects of the invention are applicable to this aspect as well.
Now couching these same problems in broader terms, the need is for--in operation of a destination or target color-presenting device--
In principle the source and destination devices may be two different kinds of printers, or two different kinds of computer monitors (e. g., one an LCD screen), or the source device a printer and the target or display screen, or one device a broadcast or tape-recorded video monitor and the other a lithography system, etc.
It will be recalled that the early, classical system of, in essence, directing source-device signals to output- or destination devices did have one technical advantage: the gamuts of the two devices were fully and perfectly superposed. Satisfying this condition ensures complete gamut usage (including vivid colors) and complete information preservation.
It is accordingly desirable in a modern system to restore some such relation--but not perceptually willy-nilly as in the early systems. Rather instead gamut superposition should be effected in a manner that somehow incorporates a perceptual frame of reference, to provide better hue control and some more-acceptable degree of lightness control than in those early systems.
In addition it is desirable to provide methodology for avoiding heavy usage of memory and processing time where not really essential. This is particularly important as to transformations within hue pages.
As can now be seen, important aspects of the technology which is used in the field of the invention are amenable to useful refinement.